Seat frame manufacturing method

ABSTRACT

A seat frame manufacturing method including: performing extrusion molding of a metal material to form a workpiece material, which has a closed section shape or a solid section shape in which a plate piece continuous in an extrusion direction protrudes in a protruding direction from a portion thereof; and performing press working, the press working including bending a part of the plate piece of the workpiece material in an out-of-plane direction of the plate piece, which is different from the protruding direction of the plate piece, to obtain a finished product.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No. 2016-090908 filed on Apr. 28, 2016, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to a method for manufacturing a seat frame.

BACKGROUND

As one of a back frame structure of a vehicle seat, there is a side frame constituting a frame-shaped back frame that is a framework of a seat back. The side frame is made by performing extrusion molding of a metal material. In the structure disclosed in JP-B-5823323, the side frame is formed by performing extrusion molding of a metal material to obtain a workpiece material, which has a closed section shape in which a plate piece continuous in an extrusion direction protrudes from a portion thereof, bending the workpiece material, and then, cutting the workpiece material into two pieces.

The side frame in the above-described structure has high bending rigidity against a bending force in an in-plane direction of the plate piece but has low bending rigidity against a bending force in an out-of-plane direction of the plate piece. In order to compensate for this disadvantage, measures such as increasing the thickness of the plate piece are required. When the thickness of the plate piece is increased, there is a problem that an increase in weight of the side frame is caused.

SUMMARY

The disclosure has been made in consideration of the above problems and an object thereof is to provide a seat frame manufacturing method in which a seat frame with a closed section shape or a solid section shape is formed by performing extrusion molding of a metal material, whereby an increase in weight is suppressed and bending rigidity in at least two directions is enhanced.

According to an aspect of the disclosure, there is provided a seat frame manufacturing method including: performing extrusion molding of a metal material to form a workpiece material, which has a closed section shape or a solid section shape in which a plate piece continuous in an extrusion direction protrudes in a protruding direction from a portion thereof; and performing press working, the press working including bending a part of the plate piece of the workpiece material in an out-of-plane direction of the plate piece, which is different from the protruding direction of the plate piece, to obtain a finished product.

Accordingly, by performing the press working, a part of the plate piece is bent in the out-of-plane direction of the plate piece, which is different from the protruding direction of the plate piece, to obtain the finished product (seat frame). In this way, the bending rigidity of the seat frame can be enhanced not only in an in-plane direction of the plate piece but also in the out-of-plane direction of the plate piece which is different from the protruding direction of the plate piece.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a seat back frame according to one embodiment of the disclosure;

FIG. 2 is an exploded perspective view of a part centered on a left side frame in the above embodiment;

FIG. 3 is a perspective view of the left side frame in the above embodiment;

FIG. 4 is a sectional view taken along line IV-IV in FIG. 3;

FIG. 5 is a sectional view taken along line V-V in FIG. 3;

FIG. 6 is a sectional view taken along line VI-VI in FIG. 3;

FIG. 7 is a perspective view of a workpiece material formed by an extrusion molding process;

FIG. 8 is a perspective view showing a state in which a cutting process is performed on a plate piece of the workpiece material;

FIG. 9 is a perspective view showing a state in which a press working process is performed on the workpiece material to which the cutting process has been performed;

FIG. 10 is a sectional view taken along the line X-X in FIG. 8, showing a state in which the workpiece material is placed in a press mold;

FIG. 11 is a sectional view taken along the line XI-XI in FIG. 8, showing a state in which the workpiece material is placed in the press mold;

FIG. 12 is a sectional view taken along the line XII-XII in FIG. 8, showing a state in which the workpiece material is placed in the press mold;

FIG. 13 is a sectional view taken along the line XIII-XIII in FIG. 9, showing a state in which the workpiece material is molded in the press mold;

FIG. 14 is a sectional view taken along the line XIV-XIV in FIG. 9, showing a state in which the workpiece material is molded in the press mold;

FIG. 15 is a sectional view taken along the line XV-XV in FIG. 9, showing a state in which the workpiece material is molded in the press mold;

FIG. 16 is a sectional view of a situation where a bending process is partially performed on the section of FIG. 14, showing a state in which the workpiece material is molded in the press mold; and

FIG. 17 is a perspective view of a workpiece material formed by an extrusion molding process in another embodiment.

DETAILED DESCRIPTION

FIGS. 1 to 16 show a seat frame manufacturing method according to one embodiment of the disclosure. The present embodiment shows an example where the disclosure is applied to a back frame (hereinafter, simply referred to as a “back frame”) of an automobile seat. In each of the figures, respective directions of an automobile when an automobile seat to which a back frame is attached is mounted on a floor of the automobile are indicated by arrows. In the following, the description relating to directions is made with reference to these directions.

As shown in FIGS. 1 and 2, a back frame 1 is configured to be assembled in a substantially inverted U shape. The back frame 1 is configured by two side frames 10 and an upper frame 2. The two side frames 10 are elongated in a longitudinal direction and form a framework of both left and right side portions of a seat back. The upper frame 2 is bridged between upper portions of both side frames 10 and form a framework of an upper portion of the seat back. Further, a lower panel 3 for increasing a structural strength of the back frame 1 is bridged and rigidly coupled between lower side portions of the both side frames 10. Here, the back frame 1 corresponds to the “framework of the seat back” in the claims.

As shown in FIGS. 1 to 6, the left and right side frames 10 are in a mirror symmetrical relationship with respect to a plane extending in a front-rear direction at a central position of a seat in a left-right direction. Accordingly, the left side frame 10 will be described as a representative. The side frame 10 includes a main body part 11 formed by processing a workpiece material W obtained by an extrusion molding of a metal material (in the present embodiment, aluminum), and a lower bracket 12 rigidly coupled to a lower end side of the main body part 11 and connected to a recliner 5. The seat frame manufacturing method of the present embodiment is a method for manufacturing the main body parts 11 in both side frames 10. Here, the main body parts 11 in both side frames 10 correspond to the “seat frame” in the claims.

As shown in FIGS. 3 to 6, the main body part 11 has a closed section part 11 a having a constant cross-sectional shape and extending in an upper-lower direction, and an open section part 11 b having a varying cross-sectional shape and extending in the upper-lower direction. The closed section part 11 a has a front surface portion 11 a 1 and a rear surface portion 11 a 2, which have a substantially rectangular shape and extend in parallel with each other in the left-right direction, a left side surface portion 11 a 3 connecting a left end portion of the front surface portion 11 a 1 and a left end portion of the rear surface portion 11 a 2, and a right side surface portion 11 a 4 connecting a right end portion of the front surface portion 11 a 1 and a right end portion of the rear surface portion 11 a 2. The length in the left-right direction of the front surface portion 11 a 1 is slightly shorter than the rear surface portion 11 a 2. The front surface portion 11 a 1 and the rear surface portion 11 a 2 are in a relationship of intersecting perpendicularly with the left side surface portion 11 a 3. That is, a rear portion of the right side surface portion 11 a 4 extends in the front-rear direction in parallel with the left side surface portion 11 a 3 and a front portion thereof obliquely extends to the front left side. In other words, the closed section part 11 a has a shape formed by rounding (chamfering) a front right corner of a rectangle in which long sides in a cross-section shape extend in the left-right direction. Further, the rounding (R shape) is performed on inner and outer corner portions of the closed section part 11 a. On a lower end portion side of the front surface portion 11 a 1, two rivet holes 11 a 11 for fastening the lower bracket 12 with rivets are provided side by side in the upper-lower direction. Further, on an upper end portion side of the front surface portion 11 a 1, one rivet hole 11 a 12 for fastening an upper bracket 4 with a rivet is provided. The upper bracket 4 connects the side frame 10 and the upper frame 2.

As shown in FIGS. 3 to 6, the open section part 11 b has the rear surface portion 11 a 2 of the closed section part 11 a, a front-rear direction surface 11 b 1 extending in parallel with the left side surface portion 11 a 3 in the front-rear direction from the portion near a corner portion between the rear surface portion 11 a 2 and the left side surface portion 11 a 3, and a left-right direction surface 11 b 2 extending so as to be curved to the right side from a rear end portion of the front-rear direction surface 11 b 1. The front-rear direction surface 11 b 1 is formed in such a way that it is long in the front-rear direction on the lower side of the side frame 10 and becomes shorter as going upward. The left-right direction surface 11 b 2 extends toward the right side perpendicularly with respect to the front-rear direction surface 11 b 1, except for the portion of the side frame 10 connected to the upper frame 2. The portion of the left-right direction surface 11 b 2 connected to the upper frame 2 is formed to be slightly inclined rearward. This is intended for conforming to the shape of the upper bracket 2. A convex portion 11 b 11 bulging to the left side is provided at the portion of the front-rear direction surface 11 b 1 adjacent to the left-right direction surface 11 b 2. The front-rear direction surface 11 b 1 is intended for enhancing the bending rigidity of the open section part 11 b in the front-rear direction. The left-right direction surface 11 b 2 is intended for enhancing the bending rigidity of the open section part 11 b in the left-right direction, together with the convex portion 11 b 11 of the front-rear direction surface 11 b 1. On an upper end portion side of the front-rear direction surface 11 b 1, one rivet hole 11 b 12 (see FIG. 2) for fastening the upper bracket 4 with a rivet is provided. Further, on a lower end portion side of the left-right direction surface 11 b 2, two rivet holes 11 b 21 are provided at positions corresponding to the rivet holes 11 a 11 of the front surface portion 11 a 1 of the closed section part 11 a. Here, the convex portion 11 b 11 corresponds to the “groove portion” in the claims.

As shown in FIGS. 1 and 2, the lower bracket 12 is a press part made of iron. An upper side portion of the lower bracket 12 is formed as a main body part connection portion 12 a in which a horizontal section is formed in a substantially inverted U shape opening toward the inner side of the seat, and a lower side portion thereof is formed as a flat plate-like recliner connection portion 12 b. The main body part connection portion 12 a has a front side surface portion 12 a 1, a rear side surface portion 12 a 2, and a left side surface portion 12 a 3 connecting the front side surface portion 12 a 1 and rear side surface portion 12 a 2. The front side surface portion 12 a 1 is provided with two rivet holes 12 a 11 for connecting the main body part 11, which are provided side by side in the upper-lower direction so as to correspond to the rivet holes 11 a 11 of the main body part 11. Further, although not shown, the rear side surface portion 12 a 2 is also provided with two rivet holes for connecting the main body part 11, which are provided side by side in the upper-lower direction so as to correspond to the rivet holes 11 b 21 of the main body part 11. The left side surface portion 12 a 3 is formed to be in close contact with the left surface of the front-rear direction surface 11 b 1 of the main body part 11 when the lower bracket 12 is connected to the main body part 11. A recliner connection hole 12 b 1 for connecting the recliner 5 is provided in the central portion of the recliner connection portion 12 b.

As shown in FIGS. 1 and 2, the upper frame 2 is a part obtained by cutting and drilling a workpiece material having a constant cross-section, which is formed by performing extrusion molding of a metal material (in the present embodiment, aluminum). A cross-sectional shape of the upper frame 2 has a closed section part 2 a disposed at an upper side and an open section part 2 b disposed below the closed section part 2 a. On a left end portion side of the rear surface portion of the closed section part 2 a, a rivet hole 2 a 1 for fastening the upper bracket 4 with a rivet is provided. Further, on a left end portion side of the rear surface portion of the open section part 2 b, a rivet hole 2 b 1 for fastening the upper bracket 4 with a rivet is provided. A pair of left and right holders 6 for inserting and holding a headrest support is connected to the front surface side of the upper frame 2 at a bilaterally symmetrical position with respect to the central position of the seat in the left-right direction.

As shown in FIGS. 1 and 2, the lower panel 3 is a part obtained by cutting and drilling a workpiece material having a constant cross-section, which is formed by performing extrusion molding of a metal material (in the present embodiment, aluminum). The lower panel 3 has a cross-sectional shape that is formed in a substantially U shape opening toward the front side. Ribs protruding toward the front side and extending in the left-right direction are provided at an opening portion of the U shape and a surface portion opposite to the opening portion. On a left end portion side of the lower panel 3, two rivet holes 3 a for fastening the lower panel 3 to the main body part 11 and the lower bracket 12 of the side frame 10 with rivets are provided. The rivet holes 3 a are provided at positions corresponding to the rivet holes 11 b 21 of the main body part 11 and the rivet holes of the rear side surface portion 12 a 2 of the lower bracket 12.

As shown in FIGS. 1 and 2, the upper brackets 4 are press products of a steel plate or an aluminum plate and are in a mirror symmetrical relationship with respect to the plane extending in the front-rear direction at the central position of the seat in the left-right direction. Accordingly, the left upper bracket 4 will be described as a representative. The upper bracket 4 has a front surface portion 4 a, a rear surface portion 4 b, a left surface portion 4 c, and an upper surface portion 4 d. On a lower end portion side of the front surface portion 4 a, a rivet hole 4 a 1 for rivet connection to the rivet hole 11 a 12 of the main body part 11 is provided. On a lower end portion side of the left surface portion 4 c, a rivet hole 4 c 1 for rivet connection to the rivet hole 11 b 12 of the main body part 11 is provided. Further, although not shown, a rivet hole for rivet connection to the rivet hole 2 a 1 and the rivet hole 2 b 1 of the upper frame 2 is provided in the rear surface portion 4 b. A reinforcing member 7 is a press product of a steel plate or an aluminum plate and has a rivet hole 7 a at the center. When the rivet hole 7 a is aligned with the rivet hole 2 b 1 of the upper frame 2 and is disposed on the inner side (front surface side) of the open section part 2 b of the upper bracket 4, the rear surface side of the reinforcing member 7 is formed in a shape in contact with an inner surface of the open section part 2 b of the upper bracket 4.

A method of assembling the back frame 1 will be described with reference to FIGS. 1 and 2. First, the main body parts 11 of the left and right side frames 10 are connected to the upper frame 2. The reinforcing member 7 is placed on the left end portion side of the upper frame 2, and the rivet hole 2 b 1 and the rivet hole 7 a are aligned. In this state, the upper bracket 4 is covered thereon. In this state, the rivet hole 2 a 1 and the rivet hole 2 b 1 of the upper frame 2 and the rivet hole (not shown) of the rear surface portion 4 b of the upper bracket 4 are aligned and rivet-connected. Then, an upper end portion side of the main body part 11 of the side frame 10 is inserted into a lower portion of the upper bracket 4 from below, and the rivet hole 11 a 12 and the rivet hole 11 b 12 of the side frame 10 and the rivet hole 4 a 1 and the rivet hole 4 c 1 of the upper bracket 4 are aligned and rivet-connected. The same is similarly applied for the right side.

Subsequently, the main body parts 11 and the lower brackets 12 of the left and right side frames 10 are connected to the lower panel 3. The rivet holes 11 a 11 and the rivet holes 11 b 21 of the main body part 11 are respectively aligned with the rivet holes 12 a 11. of the lower bracket 12 and the rivet holes of the rear side surface portion 12 a 2, and the rivet holes 11 b 21 and the rivet holes of the rear side surface portion 12 a 2 are aligned with the rivet holes 3 a of the lower panel 3. At this time, the rear surface portion of the lower panel 3 is disposed so as to be in contact with the front surface portion of the left-right direction surface 11 b 2 of the main body part 11. In this state, the front surface portion 11 a 1 of the main body part 11 and the front side surface portion 12 a 1 of the lower bracket 12 are rivet-connected, and the lower panel 3, the left-right direction surface 11 b 2 of the main body part 11 and the rear side surface portion 12 a 2 of the lower bracket 12 are rivet-connected. Meanwhile, the lower bracket 12 may be connected to the main body part 11 after the recliner 5 is previously connected to the lower bracket. Alternatively, the recliner 5 may be connected to the lower bracket 12 after the lower bracket 12 is connected to the main body part 11. The same is similarly applied to the right side. Finally, a horizontal wire 8 for supporting a back pad that is a cushion material is bridged between the both side frames 10, and a vertical wire 9 is bridged between the upper frame 2 and the lower panel 3.

A method of manufacturing the main body part 11 of each side frame 10 will be described with reference to FIGS. 7 to 16. As shown in FIG. 7, first, a metal material (in the present embodiment, aluminum) is extrusion-molded to form the elongated workpiece material W which has the closed section part Wa (11 a) and the plate piece Wb described above and has a uniform cross-sectional shape. The length of the workpiece material W obtained herein is the length of the main body part 11 of the side frame 10 in the upper-lower direction. This process corresponds to the “extrusion molding” in the claims.

Subsequently, the plate piece Wb of the extrusion-molded workpiece material W is cut along a cutting line C shown in FIG. 7. This process corresponds to the “cutting” in the claims. As a cutting means, a press may be utilized, or, a means such as a laser cutting or a wire cutting may be utilized. The shape of the workpiece material W after the cutting process is shown in FIG. 8. By this cutting process, a protruding length of the plate piece Wb is changed depending on portions, and the plate piece Wb is formed to be gradually shorter from one end side toward the other end side substantially in an extrusion-molding direction. The side having a long protruding length is formed as the lower side (the side to be connected to the lower bracket 12) of the main body part 11 of the side frame 10 through the next press working process. Simultaneously with or after this cutting process, the rivet holes 11 a 12, 11 b 12, 11 b 21 are provided in the workpiece material W.

Subsequently, the plate piece Wb of the workpiece material W to which the cutting process has been performed is formed into the open section part 11 b by press molding. The shape of the plate piece Wb of the workpiece material W after the press molding is shown in FIG. 9. This process corresponds to the “press working” in the claims. First, the workpiece material W to which the cutting process has been performed is placed between a press lower mold P1 and a press upper mold P2. This state is shown in FIGS. 10 to 12. As shown in FIG. 10, to a portion which corresponds to the lower end portion side of the main body part 11 of the side frame 10, a cavity corresponding to the open section part 11 b in FIG. 6 is formed to the portion corresponding to the plate piece Wb by the press lower mold P1 and the press upper mold P2 when the press lower mold P1 and the press upper mold P2 are closed. Further, a pressing force is not applied to the portion corresponding to the closed section part Wa. With this configuration, when the press lower mold P1 and the press upper mold P2 are closed, the plate piece Wb is molded and formed into the open section part 11 b having the front-rear direction surface 11 b 1 and the left-right direction surface 11 b 2, as shown in FIG. 13. Meanwhile, the closed section part Wa is formed into the closed section part 11 a without changing its shape.

Similarly, as shown in FIG. 11, to a portion which corresponds to a central portion of the main body part 11 of the side frame 10 in the upper-lower direction, a cavity corresponding to the open section part 11 b in FIG. 5 is formed to the portion corresponding to the plate piece Wb by the press lower mold P1 and the press upper mold P2 when the press lower mold P1 and the press upper mold P2 are closed. However, a bending portion 11 b 22 formed at the right end portion of the left-right direction surface 11 b 2 is formed in a state not bent. Further, a pressing force is not applied to the portion corresponding to the closed section part Wa. With this configuration, when the press lower mold P1 and the press upper mold P2 are closed, the plate piece Wb is molded and formed into the open section part 11 b having the front-rear direction surface 11 b 1 and the left-right direction surface 11 b 2, as shown in FIG. 14. Meanwhile, the closed section part Wa is formed into the closed section part 11 a without changing its shape. Thereafter, as shown in FIG. 16, the second pressing is performed by the press lower mold P1 and a press upper mold P3 to form the bending portion 11 b 22 at the right end portion of the left-right direction surface 11 b 2. As a result, the open section part 11 b in FIG. 5 is formed.

Similarly, as shown in FIG. 12, to a portion which corresponds to the upper end portion side of the main body part 11 of the side frame 10, a cavity corresponding to the open section part 11 b in FIG. 4 is formed at the portion corresponding to the plate piece Wb by the press lower mold P1 and the press upper mold P2 when the press lower mold P1 and the press upper mold P2 are closed. Further, a pressing force is not applied to the portion corresponding to the closed section part Wa. With this configuration, when the press lower mold P1 and the press upper mold P2 are closed, the plate piece Wb is molded and formed into the open section part 11 b having the front-rear direction surface 11 b 1 and the left-right direction surface 11 b 2, as shown in FIG. 15. Meanwhile, the closed section part Wa is formed into the closed section part 11 a without changing its shape.

The present embodiment configured as described above has the following operational effects. A part of the plate piece Wb of the workpiece material W is bent in a direction substantially perpendicular to the protruding direction of the plate piece Wb by the press working process, thereby obtaining the left-right direction surface 11 b 2. In this way, the bending rigidity of the main body part 11 can be enhanced at the front-rear direction surface 11 b 1 in the front-rear direction that is the protruding direction of the plate piece Wb and can be enhanced at the left-right direction surface 11 b 2 in the left-right direction that is a direction substantially perpendicular to the protruding direction of the plate piece Wb. Further, since the plate piece Wb is formed with respect to the closed section part Wa and the closed section part Wa has a hollow cylindrical shape, it is possible to achieve weight reduction while securing rigidity. Furthermore, since the protruding length of the plate piece Wb can be changed depending on portions by the cutting process before the press working process, the length of the front-rear direction surface 11 b 1 in the front-rear direction and/or the length of the left-right direction surface 11 b 2 in the left-right direction can be changed depending on portions. Particularly, the length of the front-rear direction surface 11 b 1 in the front-rear direction and/or the length of the left-right direction surface 11 b 2 in the left-right direction can be made longer toward the lower side (side to be connected to the lower bracket 12) where bending rigidity is required. With this configuration, since the necessary rigidity can be secured for each part with the optimum amount of material used, it is possible to achieve the weight reduction. Furthermore, since the convex portion 11 b 11 is formed in the front-rear direction surface 11 b 1, it is possible to further enhance the rigidity in the left-right direction.

FIG. 17 shows another embodiment of the disclosure. The present embodiment is different from the one embodiment in that a workpiece material W1 is used in place of the workpiece material W. The workpiece material W1 has a solid section part Wa1 in place of the closed section part Wa of the workpiece material W. As compared with the one embodiment, the present embodiment is inferior in terms of the weight reduction but can obtain the effects of improving the bending rigidity in the front-rear direction and in the left-right direction by the open section part 11 b.

Although specific embodiments have been described above, the disclosure is not limited to these appearances and configurations, and various modifications, additions and deletions can be made without changing the spirit of the disclosure. For example, the following can be exemplified.

1. In the above embodiment, the seat frame manufacturing method of the disclosure has been applied to the main body parts 11 of the side frames 10 of the back frame 1. However, the disclosure may be applied to the manufacture of various frames configuring a seat main body, such as a side frame of a cushion frame.

2. In the above embodiment, the bending direction of the plate piece Wb in the press working process is set to the inward direction of the seat. However, the disclosure is not limited thereto. For example, the rigidity in the left-right direction may be secured by bending the plate piece Wb in the outward direction of the seat.

3. In the above embodiment, the thickness of the workpiece materials W, W1 is set to be substantially equal between the closed section part Wa and the plate piece Wb. However, the disclosure is not limited thereto. For example, the thickness of the workpiece materials W, W1 may be partially changed according to the necessary rigidity, as appropriate. Particularly, it is preferable to increase the thickness of the protruding leading end portion of the plate piece Wb since the thickness can become thin at the upper portion of the main body part 11 where the bending rigidity is less required and can become thick at the lower portion of the main body part 11 where the bending rigidity is required. This is also advantageous for molding the plate piece Wb having no defects in the extrusion molding of the workpiece materials W, W1.

4. In the above embodiment, the plate piece Wb is formed on one side of the closed section part Wa. However, the disclosure is not limited thereto. For example, the plate piece Wb may be formed on both sides of the closed section part Wa. Further, the closed section part Wa may not be a complete closed section but may be provided with a narrow slit as long as it does not impair its function as the closed section.

5. Although the disclosure is applied to an automobile seat, the disclosure may be applied to a seat to be mounted on a vehicle such as an airplane, a ship or a train. 

What is claimed is:
 1. A seat frame manufacturing method comprising: performing extrusion molding of a metal material to form a workpiece material, which has a closed section shape or a solid section shape in which a plate piece continuous in an extrusion direction protrudes in a protruding direction from a portion thereof; and performing press working, the press working including bending a part of the plate piece of the workpiece material in an out-of-plane direction of the plate piece, which is different from the protruding direction of the plate piece, to obtain a finished product.
 2. The seat frame manufacturing method according to claim 1, wherein the out-of-plane direction of the plate piece is a direction substantially perpendicular to the protruding direction of the plate piece.
 3. The seat frame manufacturing method according to claim 1, wherein the extrusion molding is performed so as to form the workpiece material having the closed section shape in which the plate piece continuous in the extrusion direction protrudes in the protruding direction from the portion thereof.
 4. The seat frame manufacturing method according claim 1, further comprising: cutting the plate piece so as to change a protruding length of the plate piece depending on portions, the cutting being performed between the extrusion molding and the bending.
 5. The seat frame manufacturing method according to claim 1, wherein the extrusion molding is performed such that a thickness of the workpiece material is partially changed.
 6. The seat frame manufacturing method according to claim 1, wherein the press working includes forming a groove portion to the plate piece, the groove portion extending in a direction in which bending rigidity of the plate piece can be enhanced.
 7. The seat frame manufacturing method according to claim 1, wherein the finished product is used as a framework of a seat back of a vehicle seat. 